Add _taskqueue to MP Pool

[Sterbic]

Make the following snippet work correctly with mypy:

from multiprocessing import Pool
pool = Pool(1)
print(pool._taskqueue.empty())

[gvanrossum]

Why do you want to add an undocumented internal?

[Sterbic]

I am debugging some race conditions when submitting jobs to an MP pool and ._taskqueue is useful to verify that a job is actually sent to a worker before the next on is submitted. I need to land code using it to gather data in prod so I though I'd rather send a PR than land it with a # type: ignore.

[gvanrossum]

I don't think that argument is strong enough. Just add the # type: ignore.